1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of Related Art
Conventionally, in image forming apparatuses, it is known that even when toner weight (toner deposit) per unit area on a paper is same, density of an image which is the final output differs according to different types of papers. For example, image density becomes denser in coated papers, however, image density becomes lighter in coarse-woven papers. This is due to an influence of permeation degree of toner to a paper during fixing process.
Therefore, when an image is formed on a coarse-woven paper by using an apparatus in which the print density standard is set based on papers which are prone to increase image density such as coated papers, there is a problem that the image density becomes light.
In view of the above problem, there is suggested an technique for avoiding such problem by changing the toner deposit when such decrease in density exceeds the limit that can be solved by the density adjustment function provided in a regular image forming apparatus.
In particular, for example, JP 2009-139561 discloses a technique which focuses on the fact that the density of toner image on a coated paper is denser when same amount of toner is used for a regular paper and a coated paper when looking at the relation between toner weight and density when forming an image on a regular paper and a coated paper, and matches the density of toner images in the papers by decreasing the toner amount when coated papers are being used.
Further, JP 2004-184509 discloses a technique for easily changing (setting) the image forming condition based on a desired image density and for forming an image in the density of user's preference even when the paper types are different.
Among image forming apparatuses, an image forming apparatus which is provided with a sensor near the intermediate transfer belt in the image forming apparatus, which generates a patch image for controlling the toner amount to be deposited on a paper at a position corresponding to a position between toner images of images to be formed consecutively and performs image quality stabilizing correction processing to stabilize the image quality by reading the patch image by the sensor and feeding back the read patch image to the image parameter is known. By performing the above processing, varying of images can be inhibited while maintaining productivity of image forming.
However, in the techniques described in JP 2009-139561 and JP 2004-184509, when the image quality stabilizing correction processing is performed in a state where the toner deposit is increased in order to ensure image density, a patch image to which greater amount of toner than usual is attached is to be generated on the photosensitive drum and the intermediate transfer belt in the image forming apparatus. Therefore, such patch image is excluded from the sensitivity range of the sensor which reads density of patch images and an appropriate correction will not be performed.
If the increased toner deposit is made to return to the usual amount at the time of generation of patch image, for example, such problem can be avoided. However, the conveyance interval between consecutive papers is set by considering productivity, and there is not enough time to control so as to return the increased toner deposit to the usual amount and generate a patch image and then increase the toner deposit again during an interval between regular papers.
Therefore, when toner deposit is increased in order to ensure image density, the image quality stabilizing correction processing cannot be performed properly in such state, and as a result, there is a problem that stability in image quality cannot be ensured.